Laying of concrete kerbs, haunches and the like

ABSTRACT

A displacement detecting device in which a pivoted carrier for two permanent magnets is located in relation to two magnetic switches so that in one carrier-limit position one magnet closes one switch but in the other limit position the other magnet closes the other switch, thereby to constitute a device for sensing deviations, e.g. from a datum wire so as to operate an automatic continuous concrete laying machine.

United States Patent 1 1 Lowen et al.

LAYING OF CONCRETE KERBS, HAUNCHES AND THE LIKE lnventors: Michael David Lowen, London; Alan Reginald Mouutford, Borehamwood, 1 both of England Assignee: John Laing Research & Development Limited, London, England Filed: Sept. 17, 197 1 App]. No.5 181,320

Related US. Application Data Division of Ser. No. 836,078, June 24, 1969, Pat. No. 3,636,833.

Foreign Application Priority Data Sept. 3, 1968 Great Britain 41,939/68 Sept. 3, 1968 Great Britain 5,554/68 US. Cl. 335/206, 335/207, 94/46 AC Int. Cl. ll0lh 36/00 Field of Search 335/205-207; ZOO/61.42,

61.44; 97/46 R, 46 AC Primary Examiner-Roy N. Envall, Jr. Attorney-Steward & Steward ABSTRACT A displacement detecting device in which a pivoted carrier for two permanent magnets is located in relation to two magnetic switches so that in one carrierlimit position one magnet closes one switch but in the other limit position the-other magnet closes the other switch, thereby to constitute a device for sensing deviations, e.g. from a datum wire so as to operate an automatic continuous concrete laying machine.

11 Claims, 13 Drawing Figures F 230 206 T Q 208 2n- XHI 206 233 205 F 207 225 2'7 234 m 243 201 M 226 F--r-f- 2 Ex- E H I 222, l r Cm -F 3 223 o 2|b 2m 2 1;; 221 202 m PATENTEDJUL 3 1 ma sum as or 1 FIG. 8.

ATTORNEY n-mvmusn 3,750,0 3

SHEET 09 up ATTORNEY PATENTuJuL31 ma sum 10 or 1 FIG.|3.

ATTORNEY LAYING OF CONCRETE KERBS, HAUNCI-IES AND THE LIKE This application is a division of co-pending application Ser. No. 836,078, filed June 24, 1969, which issued Jan. 25, 1972 as US. Pat. No. 3,636,833. The invention relates to a displacement detecting device for sensing deviations from a neutral position or attitude. Such a device has particular utility in road construction equipment for continuously laying a concrete kerb, bankette, strip or haunch by a slipform process, in which the surface level of the laid concrete is controlled to follow a predetermined datum line.

According to the invention described in our Application U.S. Ser. No. 836,078 a machine for continuously the laying a concrete kerb, strip, bankette or haunch in accordance with a predetermined datum line provided by a taut wire or the like, comprises a mobile carriage having a hopper from which the concrete is discharged between slipforms and levelled, where-in the depth of the laid concrete is continuously controlled by tools carried on a subframe whose vertical, height is continuously adjusted by means responsive to sensing members which mechanically engage the wire.

The present invention relates to displacement detecting devices of a nature which can be used in the haunch laying equipment described above, although not confined to such use, an object of the invention being to provide a device in which switches are selectively operated in response to the displacement of a carrier control element in either direction from a null position. The device is primiarily intended for the control of a component to maintain it in a predetermined or null position or attitude relative to a datum. Such component may, for example, be an assembly of levelling and compacting tools of the concrete haunch or kerb laying machine used in road construction, the datum being represented by some fixed device such as a taut wire. Another object of the present invention is to provide displacement detecting devices .in which the control of forward and reverse circuits is performed by magnetic fields acting on magnetically responsive switch elements, thereby eliminating direct mechanical contact between the movable carrier or control element and the switches themselves. This results in higher sensitivity to small deflecting forces acting on the control element. I

According to the present invention, the control eleposition and for'moving it to either of its limit positions,

ment is constituted by a magnet-carrying member' mounted for pivotal deflection relative to a switchcarrying member between two limit positions, said magnit carrying member having a pair of permanent magnets mounted thereon, each magnet being adapted to effect operation of a pair of magnetically operated switches mounted on. the s witchcarrying member in spaced relationship to each other andon opposite sides of an intermediate neutral or null position.

Oneof the carrying members is movable relative to the other from a neutral position to either of two limiting positions, such that at one of the limit positions, the contacts of one of the switches are closed by one of the magnets, while at the other limit position the second switch is closed by the other magnet. A sensing element, which has a neutral attitude, is associated with the one carrying member for positioning it in its neutral when the sensing element is deviated from its neutral attitude, as for example by the aforementioned datum wire.

Each pair of switch contacts-may be normally closed and opened by the action of its magnet, but generally a magnet, upon displacement to a position for effective actuation of its switch contacts,,causes closure thereof, the switch contacts being spring biased to the normally open position. It will be understood that the switch contacts may operate relays, contactors or the like.

Advantageously, the magnet-carrier is of inverted U- shape and has the magnets mounted adjacent the free ends of the respective limbs of the U, and pivots are mounted on the limbs adjacent their roots, the pivots being coaxial in the plane of the axes of the two limbs of the U.

The invention will now be described, by way of example, with reference to theaccompanying drawings in which:-

FIG. 1 is a side elevation, omitting some parts for clarity, of a concrete haunch laying machine, with which the displacement detecting device of the present invention isparticularly applicable;

FIG. 2 is a plan view of the machine shown in FIG.

FIG. 3 is an end elevation on the arrow III of FIG. I with some parts omitted for clarity;

FIG. 4 is a diagrammatic side view of the datum wire and its supports;

' FIG. 5 is a section on the line A-A inFIG. 4;

FIG. 6 is a side elevation of a level-control unit embodying the invention;

FIG. 7 is a part-sectional plan of FIG. 6;

FIG. 8 shows a detail of FIG. 6,

FIGS. 9, l0 and 11 show a variant of a level-control unit, each view corresponding to that of'FIGS. 6, 7 and 8, respectively;

FIG. 12 shows a scrap perspective of a different form of subframe from that employed in FIGS. 1-3, and

FIG. 13 shows a sensing device embodying the invention, which is especially suitable for use with the apparatus of FIG. 16. X

The machine comprises a mobile carriage 10 mounted on a pair of endless track units, 11, 12, to

which it is connected by a three-point support. The

wheels. The motors 21, 22 are fed with pressurised fluid from pumps 29, 30 which are driven from an intemal combustion engine 31 through belt and pulley drives 32, 33, the common drive shaft 34 of which is mounted in bearings 35, 36 and coupled by a coupling 37 to the crankshaft of the engine. The engine also drives an electric generator 39 through a belt and pulley drive 40, and further hydraulic pumps referred to later.

At the front end of the machine is a hopper 42 having a discharge opening 43 (FIG. 2) through which the concrete is deposited between slipforms 44. The level of the deposited concrete is continuously controlled by means of a vertically adjustable. gate 45, against which bears a wiper blade 46 mounted on the backplate of the hopper 42 to prevent concrete from entering the gate guide opening. Behind the gate 45 are a tamper and smoothing tools (not shown) which are supported from a common subframe 49, the smoothing plate being connected by rubber blocks for absorbing vibrations. A hydraulic motor reciprocates a tamper to follow an approximately vertical path.

Height adjustment of the gate 45-, tamper and smoothing plate is by means of a pair of reversible electric motors 61, 62 energized by a generator.

Each motor 61, 62 is controlled by its own control unit or displacement detecting device 74, 75 mounted on the subframe, which is actuated by a respective sensing or detector element 76, 77 engaginga taut 77) remains in contact with the datum wire, the spindle 93 will be deflected by relative, movements between the sensing element and control unit casing, and the carrier arm 90 will be tilted correspondingly. The extent of tilting is limited by adjustable stops 103. In order to reduce vibration of the carrier, the latter carries a damper blade 104 mounted at the end of a spindle 105 and dipping into a bath 106 of silicone liquid within the block "107. Each limb of the carrier arm 90 carries a permanent magnet 108, 109 which are spaced far enough apart so that they do not interact with each datum wire 78. This datum wire is positioned as near the centre-line of the slipforms as is practicable in order to minimise positional errors of the tamper and smoothing tools due to canting or tilting of the machine. The wire 78 is thus straddled by the machine during the haunch-laying operation as shown in FIGS. l-3. 11

Basically, each sensing element comprises an arm pivoted about a horizontal axis and biased into engagement with the underside of the wire. Two permanent magnets are mounted at the inner end of the arm, and 6 these magnets operate reed switches to energise the motor in one or other direction according to whether the arm lies above or below a centre position in which the reed switch is open and the motor not energised.

FIGS. 4and'5 illustrate the datum wire. Thewire 78 is suspended from stakes 79 spaced at 25-foot intervals and stretched to the required tension between end posts 80, 81, the wire passing rounda pulley 82 on the post 80 and the pulley incorporating a one-way sprocket which is weighted by a weight 83. Each stake 79 carries a horizontal arm 84, whose vertical position is accurately set in accordance with the required datum level, and a plate 85 is set at the required horizontal position on this arm. The wire 78 passes through a notch ings on a scale. i In a modification steering is carried'out automatically in the bottom of the plate 85 and is retained therein by a light tension spring suspended from the arm 84. In FIG. 4 the ground level is indicated at 86, and the required level of the haunch is indicated at 87.

Referring now to FIGS. 6-8,each displacement detecting device 74, 75 comprises a mild steel casing 88 having an insulating block 107 rigidly mounted on a plate 89. Achannel-shaped aluminium carrier arm 90 is pivotally mounted on this block by needle bearings 91 whose pointed ends engage with bearing surfaces provided by hardened steel inserts 92 in the block. Secured tothe carrier'arm 90 in alignment with the axis of the pivot is spindle 93 projecting through an opening 94 in the casing. As shown in FIG. 7, the spindle 93 carriesa collar 95 which supports a plate 96, and a hollow cylinder 97 is secured to the other end of the plate.

The arm of the sensing element 76 (or 77), which is bitherein by a tension spring 100. To permit adjustment I of the weight 98, its is slotted at 101 and clamped in position by a clamping nut 102. i it will be seen that, as the machine follows undulations in the ground while the sensing element 76 (or I fall, but in the laying of substantially wider strips it is v other. Each magnet 108, 109 cooperates with and actuates a respective reed switch 110, 11 l magnetically, the motor 61 (or 62) being energised for rotation in one or other direction according to which of the reed switches is closed. Several combinations of reed switches and magnets are possible, but it is preferred to use a system in which the motor is de-energised (i.e. both switches open) for the shortest practicable travel of the carrier through its centre position, since this provides the most sensitive control.

The machine is steered by controlling the drive to the track units differentially. A bridge 112 (FIG. 2) carries levers 113, 114 which control the'outputs of the pumps 29, 30 through connecting rods and valves (not shown). In practice these control levers provide a coarse adjustment for both pump units, and one is normally preset to a required setting; a fine control lever 115 operates a cam unit 116 which optionally interlocks the levers 113, 114 and thereby moves the levers differentially to provide a fine adjustment for one of the pump units. The lever 115 is continuously manipulated to provide the necessary differential adjustments for the other pump unit. If the machine is steered manually, the operator sights the datum wire by means of mirrors so as to keep the wire between specified markthe machine being controlled to follow a predetermined course set by the datum wire. ln-such a case the coarse control lever is preset as for manual steering and a servo control mechanism (not shown) provides fine adjustment of one pump. The servo mechanism is actuated in accordance with the position of a pivotal probe biased laterally against the wire and preferablyconstructed in similar manner to the elements 76, 77 for level control.

When the machine is being used to lay a narrow strip of concrete, such as a haunch of twelve inches width, it is not normally necessary to compensate for cross usually necessary to provide additional control means for keeping the surface of the strip level.

The machine described may also be provided'witha feeding device (not shown) in the form of'asmall high speed electric-conveyor with a receiving hopper at its outboard end. The feeding device would be suspended from an A-frame over the main hopper 42 of the ma-' chine and extend forwards so that the receiving hopper is ahead of the machine. The inboard end of the feeding device would be pivoted horizontally and vertically so as to allow, the' receiving hopper to be positioned on either side of the datum wireas required. '9

Although the variant shown in FIGS 9, 10 and 11 will be generally understood from the description of the broadly similar FIGS. 6, 7 and 8 it will for the sake of completeness now be described in more detail.

The displacement detecting device shown in FIGS. 9-11 includes an upright supporting frame or bracket 201 from which projects a non-magnetic plate 202 serving as a mount for a block 203 of insulating material secured in position by screws 204.

Near its upper. end the block 203 carries, on opposite sides hardened steel inserts 205 having heads or flanges 206 formed with cup-shaped or conical indentations to form seatings for coaxial needle bearing pivots 207, 217 pivotally supporting an inverted U-shaped yoke or carrier 208 of aluminium or other non-magnetic material, similar to the carrier arm 90 of the unit shown in FIGS. 6-9. As shown, the pivots 207, 217 are axially adjustable, being screwed through threaded holes in the upper opposite limbs 209, 210 of the carrier and secured in their adjusted positions by lock nuts 211.

The carrier 208 has a neutral position, normally a position in which the limbs 209 and 210 hang vertically midway between two limit positions determined by engagement of the arm 209 with stop screws 212, 213 mounted in a U-shaped support 215 bolted at 214 to a side of the block 203, the screws 212 and 213 being locked in their adjusted positions by lock nuts. Mounted on the carrier are two permanent magnets 216 and 218 mounted on the limbs 209, 210 respectively near their lower ends in spaced relationship to each other so that they do not interact.

Each of the magnets is associated with a respective magnetically actuated reed switch. Thus the magnet 216 is associated with and is adapted to effect operation of the contacts of a switch 219 and the magnet 218 is associated with and is adapted to effect, operation of the contacts of a switch 220. Each of the switches 219 and 220 is illustrated in the drawins consists of at least a pair of contacts, one of which is movable by an armature, sealed into a glass or other non-magnetic gas impervious envelope. Each is supported by its end connections in a pair of sockets 221 on an adjustable carrier plate 222 adjustably clamped on the block 203 by means of screws 223. The sockets 221 supporting a switch also serve as terminals for that switch, and the bracket 201 is formed with an aperture fitted with a grommet 224 of insulating material for the passage of leads to the switch terminals.

Mid-way between the limbs 209 and 210 the yoke carries a depending central rod 225 projecting into chamber 226 formed in the block 203 and containing a suitable liquid such as a silicone. At its lower end the rod carries a damper vane 227 submerged in the liquid.

The rod 225 passes through a threaded hole in the carrier 208 and serves as a bolt for clamping on the carrier 208 a biasing arm 230. The arm is slotted at 231 for adjustment of the bias leading and projects at right angles to the pivot axis of the carrier 208 so as to bias the carrier by gravity towards one of its limit positions. The

washer 234 in an aperture 233 in the bracket 201.

Clamped to the spindle 232 on the outer side of aperture" 233 in the bracket 201 is a crank am 235 which includes a split collar 236 clamped onto the spindle by screws 237. The crank arm carriesat its outer end a hollow cylinder 238 in which seats an acorn fitting 239 retained therein by a tension spring 240. A sensing rod 241 projects'from-and is fixed in the acorn fitting. The whole switching unit is enclosed in a sealed casing 243.

i In use, the sensing rod 241 is biased by the biasing arm 230 against a control element such as a taut datum wire in the'direction of length of which a vehicle carrying the switching device moves. in such a case, deviation of the bracket 201 from a desired contour defined by the'datum wire will be accompanied by angular deflection of the crank arm 235 under the influence of the sensing rod 241 and corresponding rocking of the carrier 208. As the latter moves from its neutral position through a predetermined small angle towards the one or the other of its two limit positions, one or the other of the switches 219 and 220 is actuated by proximity of, and under the influence of, the associated permanent magnet 216 and 218.

Should the sensing rod 241 encounter an obstruction, the rod deflects through rotation of the acorn fitting 239 relative to the cylinder 238 against the biasing force of the spring 240 and so minimises the danger of damage to the arm 241 or another part'of the switching device. Moreover, the acron fitting limits the force exerted on the crank arm 241 by the datum device after the arm 209 has engaged a stop 212 or 213.

Several combinations of reed switches and magnets are possible, but it is preferred to use a system in which a motor is de-energised (i.e. both switches 219, 220 open) for the shortest practicable travel of the carrier 208 through its neutral or'central position, since this provides the most sensitive control.

Although in the drawings the magnets 216, 218 and switches 219, 220 are shown with their axes parallel in the horizontal plane, it is possible to obtain variations in the response characteristics of the device by modifying these spatial relationships. For example, the axes of the magnets can be rotated through in the horizontal plane, or the axes of the switches can be rotated through 90 in the vertical plane.

By a small modification, the sensing device may be arranged to control a servo mechanism or the like in order to govern movement of a vehicle in a horizontal plane. To that end the sensing rod 241 is so positioned that, withthe carrier 208 in its neutral position, the rod extends vertically from the spindle 232. The switching device may be mounted at the front of a vehicle and in operation the biasing member 230 acts to bias the sensing rod against a side of a datum device defining the desired course of the vehicle. Deviation from that course is accompanied by rocking of the carrier 208 towards one or other of its limit positions and corresponding energization of a servo mechanism or the like in the forward or reverse direction corrects the course followed by the vehicle. v The sensing device may be adapted to regulate the angular posiion about an axis of a body which is free to rock about that axis. To this end the biasing member 230 is removed, a pendulum is coupled to the spindle 232 and the switching device is mounted to rock with the said body so that, when the body is in the desired angular position, the pendulum hangs vertically and the carrier 208 is in its neutral position, whilst rocking of the body from the desired angular position is accompanied by corresponding rocking of the carrier under the influence of the pendulum relatively to the bracket 201 and parts secured thereto. Consequently one of the limits 219 and 220 is operated to effect energisation of a servo mechanism or the like to operate in a crresponding direction to correct the angular position of the body.

Referring now to FIG. 12 of the drawings, a kerb or haunch laying machine of the kind described with reference to FIGS. 1 to 3 consists of a frame or chassis (indicated fragmentarily at 370) supported on wheels or tracks (not shown) and carrying a hopper 371 for concrete, slip forms 372 which support the concrete discharged from the hopper, and a sub-frame 373 carrying smoothing and tamping tools T. These in conjunction with an adjustable gate 374 controlling the outlet from the hopper 371, govern the level of the top surface "of the haunch in accordance with signals from a pair of sensing devices 375, 376, each having its sensing tip 341 in engagement with a taut wire or similar datum 346 for the haunch. This wire is carried on levelling posts 347.

The devices 375, 376 are carried on respective ends of a rigid platform 377 suspended from the chassis 370 by links 378 and crank arms 379 clamped on respective front and rear torque tubes 380, 381. The latter are mounted in bearings 382 on the chassis 370. Similar links 383 and arms 384 couple each torque tube 380, 381 to the respective end of the subframe 373. Thus, vertical displacements of the sub-frame relative to the chassis 370 are repeated exactly by the platform 377 so that the necessary follow-up action is applied to the sensors 375, 376 as they detect deviations of the subframe 373 from the level set by the datum wire 346. The correction signals from the sensors 375, 376 are fed to corresponding level control motors 385, 386

which adjust the height of the respective ends of the sub-frame 373.

As will be understood, there are many other alternative practical applications of the present invention.

The sensors 375, 376 may be constructed in the man'- ner described with reference to and illustrated in FIGS. 6 to 8, or 9 to 11 for example but FIG. 13 of the accompanying drawings illustrates a modification particularly suitable for the arrangement of FIG. 12 in which a straight arm or carrier bar 350 is pivoted centrally at 351 and has a magnet 352, 353 mounted transversely at each end. The pivot 351 is secured on a rigid backplate 354 adapted to be mounted in a vertical plane. A feeler arm 355 is fixed at an angle to the carrier bar 350 and has a balanced sensing flap 356 pivoted thereto at its free end. The pivot 357 of the sensing flap is located at its centre of length, and the flap is biased by a spiral spring 358 against an adjustable stop 359, the spring 358 normally holding the sensing flap 356 in contact with the stop 359. The carrier bar and feeler arm assembly is balanced about the pivot 351 by a balance weight 360. A pair of adjustable stops 361 are mounted on the backplate 354 to limit the angular deflection of the carrier bar 350.

Adjacent each magnet 352, 353 are mounted respective magnetically operated .reed switches 362, 363. These switches are mounted on base plates 364 which are vertically adjustable for control of the instants of opening or closing in terms of angular deflection of the carrier bar 350.

In operation, the subframe 373 is first set to the required finished height of the kerb to be laid. The height of the datum wire 346 relativethereto is accurately known, and the platform 377 is set to a height such that the carrier 350 is in its mean or neutral position when the tip of the sensing flap 356 rests under normal static load conditions on the datum wire.

As the machine 370 move forward, following the datum wire 346, the flap 356 is held in contact with the wire by the resultant moment of the carrier and feeler arm assembly determined by the adjustment of the balance weight 360, and variations in the height of the chassis 370 relative to the datum will cause the flap to raise or lower the feeler arm 355 about the pivot 351. This deflects the magnet carrier 350 away from its new tral (horizontal) position, thus causing the reed switches 372, 363 to be selectively opeated to energise their respective motor 385, 386 in the required sense to raise or lower the subframe 373.

The subframe raises or lowers the platform 377 in synchronism until the magnet carrier 350 resumes its neutral position. Thus the height of thekerb being laid is maintained within close tolerances of that represented by the datum wire 346.

Although in the drawings the magnets 352, 353 and switches 362, 363 are shown with their axes parallel in the vertical plane, it is possible to obtain variations in the response characteristics of the device by modifying these spatial relationships. For example, the axes of the magnets can be rotated through 90 in the vertical plane, or the axes of the switches 'can be rotated through 90 in the vertical plane.

By a small modification, the sensing deivce may be arranged to control a servo mechanism or the like in order to govern movement of a vehicle in a horizontal plane. To that end the pivot 357 is mounted vertically and the sensing flap 356 is positioned beneath the feeler arm 355 so as to pivot in response to lateral forces. The sensing device may be mounted at the front of a vehicle and in operation the flap 356 rests against the side of a datum device defining the desired course of the vehicle. Deviation from that course is accompanied by rocking of the carrier 350 towards one or other of its limit positions and corresponding energisation of a servo mechanism or the like in the forward or reverse direction corrects the course followed by the vehicle.

The switching device may be adapted to regulate the angular position about an axis of a body which is free to rock about that axis. To this end the balance weight 360, feeler arm 355, and parts attached thereto are removed, a pendulum is rigidly coupled to the carrier 350 and the adjustable stops 361 are repositioned to limit the excursions of the pendulum. The sensing device is mounted to rock with the said body so that, when the body is in the desired angular position, the pendulum hangs vertically and the carrier 350 is in its neutral position, whilst rocking of the body from the desired angular position is accompanied by corresponding rocking of the carrier under the influence of the pendulum relatively to the backplate 354. Consequently, one of the switches 362 and 363 is operated to effect energisation of a servo mechanism or the like to operate in a corresponding direction to correct the angular position of the body. I l

The carrier bar 350 is shown in FIG. 13 in a horizontal mean or neutral position, but any angle to the horizontal can be chosen as the neutral, including thevertical position.

Several combinations of reed switches and magnets are possible, but it is preferred to use a system in which a motor is de-energised (Le. both switches 362, 363 open) for the shortest practicable travel of the carrier 350 through its neutral or central position, since'this provides the most sensitive control.

We claim:

1. A displacement detecting device for sensing deviaa pair of magnetically operated switches mounted on said switch-carrying member, I g ,a pair of permanent magnets mounted on said magnet-carrying member and disposed sufficiently remote from each other to be free of interaction, such that at one of said limit positions a first of said magnets closes one of said switches and at the other said limit position the second of said magnets closes the other said switch, and t a sensing element associated with one of said members for positioning said members in said neutral position relative to each other when said sensing element is in a neutral attitude and for moving said one member to either of said limit positions when said sensing element is deviated from its neutral attiutde. I 2. A device as claimed in claim 1, wherein said magnet-carrying member is supported on a pivot located intermediate its ends and said sensing element is attached to said magnet-carrying member. 3. A device as claimed in claim 2 which further includes biasing means associated with said magnet carrying member for biasing said sensing element into engagement with a datum wire.

4. A device as claimed in claim 3, wherein said biasing means is an adjustable gravitational biasing means.

5. A device as claimed in claim 2 wherein said sensing element is attached to said magnet-carrying member by a crank arm fixed to rotate with said.

6. A device as claimed in claim 5, wherein said sensing element is connected to said crank by a yieldable coupling which yields to an abnormal deflecting force.

7. A device as claimed in claim 1, include damping means to oppose oscillation of said magnet-carrying member.

8. A device as claimed in claim 1, wherein said magnet-carrying member is mounted on said pivot transverse to the paths of travel of said magnets and wherein said magnets are mounted on opposite sides of said pivot and equidistant therefrom.

9. A device as claimed in claim 1, wherein said magnet-carrying member is of an inverted U-shape having a magnet at each free end and the pivot axis is in the plane of the U 10. A device as'claimed in claim 9, wherein pivot means are located on a central block within the U and wherein each switch is mounted on said block opposite the limit position of its associated magnet.

11. A device as claimed in claim 10, comprising a chamber for damping liquid within said.block and a damper vane connected to said magnet-carrying memberextending with said chamber.

s w a e 

1. A displacement detecting device for sensing deviations from a neutral position or attitude, comprising a switch-carrying member, a magnet-carrying member, said members being mounted for pivotal movement relative to each other from a neutral position to either of two limiting positions, a pair of magnetically operated switches mounted on said switchcarrying member, a pair of permanent magnets mounted on said magnet-carrying member and disposed sufficiently remote from each other to be free of interaction, such that at one of said limit positions a first of said magnets closes one of said switches and at the other said limit position the second of said magnets closes the other said switch, and a sensing element associated with one of said members for positioning said members in said neutral position relative to each other when said sensing element is in a neutral attitude and for moving said one member to either of said limit positions when said sensing element is deviated from its neutral attiutde.
 2. A device as claimed in claim 1, wherein said magnet-carrying member is supported on a pivot located intermediate its ends and said sensing element is attached to said magnet-carrying member.
 3. A device as claimed in claim 2 which further includes biasing means associated with said magnet-carrying member for biasing said sensing element into engagement with a datum wire.
 4. A device as claimed in claim 3, wherein said biasing means is an adjustable gravitational biasing means.
 5. A device as claimed in claim 2 wherein said sensing element is attached to said magnet-carrying member by a crank arm fixed to rotate with said.
 6. A device as claimed in claim 5, wherein said sensing element is connected to said crank by a yieldable coupling which yields to an abnormal deflecting force.
 7. A device as claimed in claim 1, include damping means to oppose oscillation of said magnet-carrying member.
 8. A device as claimed in claim 1, wherein said magnet-carrying member is mounted on said pivot transverse to the paths of travel of said magnets and wherein said magnets are mounted on opposite sides of said pivot and equidistant therefrom.
 9. A device as claimed in claim 1, wherein said magnet-carrying member is of an inverted U-shape having a magnet at each free end and the pivot axis is in the plane of the U.
 10. A device as claimed in claim 9, wherein pivot means are located on a central block within the U and wherein each switch is mounted on said block opposite the limit position of its associated magnet.
 11. A device as claimed in claim 10, comprising a chamber for damping liquid within said block and a damper vane connected to said magnet-carrying member extending with said chamber. 